Published video coding standards include ITU-T H.261, ITU-T H.263, ISO/IEC MPEG-1, ISO/IEC MPEG-2, and ISO/IEC MPEG-4 Part 2. These standards are herein referred to as conventional video coding standards.
Video Communication Systems
Video communication systems can be divided into conversational and non-conversational systems. Conversational systems include video conferencing and video telephony. Examples of such systems include ITU-T Recommendations H.320, H.323, and H.324 that specify a video conferencing/telephony system operating in ISDN, IP, and PSTN networks respectively. Conversational systems are characterized by the intent to minimize the end-to-end delay (from audio-video capture to the far-end audio-video presentation) in order to improve the user experience.
Non-conversational systems include playback of stored content, such as Digital Versatile Disks (DVDs) or video files stored in a mass memory of a playback device, digital TV, and streaming. A short review of the most important standards in these technology areas is given below.
A dominant standard in digital video consumer electronics today is MPEG-2, which includes specifications for video compression, audio compression, storage, and transport. The storage and transport of coded video is based on the concept of an elementary stream. An elementary stream consists of coded data from a single source (e.g. video) plus ancillary data needed for synchronization, identification and characterization of the source information. An elementary stream is packetized into either constant-length or variable-length packets to form a Packetized Elementary Stream (PES). Each PES packet consists of a header followed by stream data called the payload. PES packets from various elementary streams are combined to form either a Program Stream (PS) or a Transport Stream (TS). PS is aimed at applications having negligible transmission errors, such as store-and-play type of applications. TS is aimed at applications that are susceptible of transmission errors. However, TS assumes that the network throughput is guaranteed to be constant.
The Joint Video Team (JVT) of ITU-T and ISO/IEC has prepared a standard published as ITU-T Recommendation H.264 and ISO/IEC International Standard 14496-10 (MPEG-4 Part 10). The standard is referred to as the JVT coding standard in this paper, and the codec according to the draft standard is referred to as the JVT codec.
The codec specification itself distinguishes conceptually between a video coding layer (VCL), and a network abstraction layer (NAL). The VCL contains the signal processing functionality of the codec, things such as transform, quantization, motion search/compensation, and the loop filter. It follows the general concept of most of today's video codecs, a macroblock-based coder that utilizes inter picture prediction with motion compensation, and transform coding of the residual signal. The output of the VCL are slices: a bit string that contains the macroblock data of an integer number of macroblocks, and the information of the slice header (containing the spatial address of the first macroblock in the slice, the initial quantization parameter, and similar). Macroblocks in slices are ordered in scan order unless a different macroblock allocation is specified, using the so-called Flexible Macroblock Ordering syntax. In-picture prediction is used only within a slice.
The NAL encapsulates the slice output of the VCL into Network Abstraction Layer Units (NAL units or NALUs), which are suitable for the transmission over packet networks or the use in packet oriented multiplex environments. JVT's Annex B defines an encapsulation process to transmit such NALUs over byte-stream oriented networks.
The optional reference picture selection mode of H.263 and the NEWPRED coding tool of MPEG-4 Part 2 enable selection of the reference frame for motion compensation per each picture segment, e.g., per each slice in H.263. Furthermore, the optional Enhanced Reference Picture Selection mode of H.263 and the JVT coding standard enable selection of the reference frame for each macroblock separately.
Reference picture selection enables many types of temporal scalability schemes. FIG. 1 shows an example of a temporal scalability scheme, which is herein referred to as recursive temporal scalability. The example scheme can be decoded with three constant frame rates. FIG. 2 depicts a scheme referred to as Video Redundancy Coding, where a sequence of pictures is divided into two or more independently coded threads in an interleaved manner. The arrows in these and all the subsequent figures indicate the direction of motion compensation and the values under the frames correspond to the relative capturing and displaying times of the frames.
Transmission Order
In conventional video coding standards, the decoding order of pictures is the same as the display order except for B pictures. A block in a conventional B picture can be bi-directionally temporally predicted from two reference pictures, where one reference picture is temporally preceding and the other reference picture is temporally succeeding in display order. Only the latest reference picture in decoding order can succeed the B picture in display order (exception: interlaced coding in H.263 where both field pictures of a temporally subsequent reference frame can precede a B picture in decoding order). A conventional B picture cannot be used as a reference picture for temporal prediction, and therefore a conventional B picture can be disposed of without affecting the decoding of any other pictures.
The JVT coding standard includes the following novel technical features compared to earlier standards:                The decoding order of pictures is decoupled from the display order. The picture number indicates decoding order and the picture order count indicates the display order.        Reference pictures for a block in a B picture can either be before or after the B picture in display order. Consequently, a B picture stands for a bi-predictive picture instead of a bi-directional picture.        Pictures that are not used as reference pictures are marked explicitly. A picture of any type (intra, inter, B, etc.) can either be a reference picture or a non-reference picture. (Thus, a B picture can be used as a reference picture for temporal prediction of other pictures.)        A picture can contain slices that are coded with a different coding type. In other words, a coded picture may consist of an intra-coded slice and a B-coded slice, for example.        
Decoupling of display order from decoding order can be beneficial from compression efficiency and error resiliency point of view.
An example of a prediction structure potentially improving compression efficiency is presented in FIG. 3. Boxes indicate pictures, capital letters within boxes indicate coding types, numbers within boxes are picture numbers according to the JVT coding standard, and arrows indicate prediction dependencies. Note that picture B17 is a reference picture for pictures B18. Compression efficiency is potentially improved compared to conventional coding, because the reference pictures for pictures B18 are temporally closer compared to conventional coding with PBBP or PBBBP coded picture patterns. Compression efficiency is potentially improved compared to conventional PBP coded picture pattern, because part of reference pictures are bi-directionally predicted.
FIG. 4 presents an example of the intra picture postponement method that can be used to improve error resiliency. Conventionally, an intra picture is coded immediately after a scene cut or as a response to an expired intra picture refresh period, for example. In the intra picture postponement method, an intra picture is not coded immediately after a need to code an intra picture arises, but rather a temporally subsequent picture is selected as an intra picture. Each picture between the coded intra picture and the conventional location of an intra picture is predicted from the next temporally subsequent picture. As FIG. 4 shows, the intra picture postponement method generates two independent inter picture prediction chains, whereas conventional coding algorithms produce a single inter picture chain. It is intuitively clear that the two-chain approach is more robust against erasure errors than the one-chain conventional approach. If one chain suffers from a packet loss, the other chain may still be correctly received. In conventional coding, a packet loss always causes error propagation to the rest of the inter picture prediction chain.
Two types of ordering and timing information have been conventionally associated with digital video: decoding and presentation order. A closer look at the related technology is taken below.
A decoding timestamp (DTS) indicates the time relative to a reference clock that a coded data unit is supposed to be decoded. If DTS is coded and transmitted, it serves for two purposes: First, if the decoding order of pictures differs from their output order, DTS indicates the decoding order explicitly. Second, DTS guarantees a certain pre-decoder buffering behavior provided that the reception rate is close to the transmission rate at any moment. In networks where the end-to-end latency varies, the second use of DTS plays little or no role. Instead, received data is decoded as fast as possible provided that there is room in the post-decoder buffer for uncompressed pictures.
Carriage of DTS depends on the communication system and video coding standard in use. In MPEG-2 Systems, DTS can optionally be transmitted as one item in the header of a PES packet. In the JVT coding standard, DTS can optionally be carried as a part of Supplemental Enhancement Information (SEI), and it is used in the operation of an optional Hypothetical Reference Decoder. In ISO Base Media File Format, DTS is dedicated its own box type, Decoding Time to Sample Box. In many systems, such as RTP-based streaming systems, DTS is not carried at all, because decoding order is assumed to be the same as transmission order and exact decoding time does not play an important role.
H.263 optional Annex U and Annex W.6.12 specify a picture number that is incremented by 1 relative to the previous reference picture in decoding order. In the JVT coding standard, the frame number coding element is specified similarly to the picture number of H.263. The JVT coding standard specifies a particular type of an intra picture, called an instantaneous decoder refresh (IDR) picture. No subsequent picture can refer to pictures that are earlier than the IDR picture in decoding order. An IDR picture is often coded as a response to a scene change. In the JVT coding standard, frame number is reset to 0 at an IDR picture.
H.263 picture number can be used to recover the decoding order of reference pictures. Similarly, the JVT frame number can be used to recover the decoding order of frames between an IDR picture (inclusive) and the next IDR picture (exclusive) in decoding order. However, because the complementary reference field pairs (consecutive pictures coded as fields that are of different parity) share the same frame number, their decoding order cannot be reconstructed from the frame numbers.
The H.263 picture number or JVT frame number of a non-reference picture is specified to be equal to the picture or frame number of the previous reference picture in decoding order plus 1. If several non-reference pictures are consecutive in decoding order, they share the same picture or frame number. The picture or frame number of a non-reference picture is also the same as the picture or frame number of the following reference picture in decoding order. The decoding order of consecutive non-reference pictures can be recovered using the Temporal Reference (TR) coding element in H.263 or the Picture Order Count (POC) concept of the JVT coding standard.
The draft RTP payload of the JVT codec (S. Wenger, et. al, “RTP Payload Format for H.264 Video,” draft-ieff-avt-rtp-h264-11.txt, August 2004) specifies three packetization modes: single NAL unit mode, non-interleaved mode, and interleaved mode. In the single NAL unit and non-interleaved modes, the decoding order of NAL units is identical to their transmission order. In the interleaved packetization mode, the transmission order of NAL units is allowed to differ from the decoding order of the NAL units. Decoding order number (DON) is a field in the payload structure or a derived variable that indicates the NAL unit decoding order. The DON value of the first NAL unit in transmission order may be set to any value. Values of DON are in the range of 0 to 65535, inclusive. After reaching the maximum value, the value of DON wraps around to 0. The decoding order of two NAL units in the interleaved packetization mode is determined as follows. Let DON(i) be the decoding order number of the NAL unit having index i in the transmission order. Function don_diff(m,n) is specified as follows:If DON(m)==DON(n), don_diff(m,n)=0If (DON(m)<DON(n) and DON(n)−DON(m)<32768), don_diff(m,n)=DON(n)−DON(m)If (DON(m)>DON(n) and DON(m)−DON(n)>=32768), don_diff(m,n)=65536−DON(m)+DON(n)If (DON(m)<DON(n) and DON(n)−DON(m)>=32768), don_diff(m,n)=−(DON(m)+65536−DON(n))If (DON(m)>DON(n) and DON(m)−DON(n)<32768), don_diff(m,n)=−(DON(m)−DON(n))
A positive value of don_diff(m,n) indicates that the NAL unit having transmission order index n follows, in decoding order, the NAL unit having transmission order index m. When don_diff(m,n) is equal to 0, then the NAL unit decoding order of the two NAL units can be in either order. For example, when arbitrary slice order is allowed by the video coding profile in use, all the coded slice NAL units of a coded picture are allowed to have the same value of DON. A negative value of don_diff(m,n) indicates that the NAL unit having transmission order index n precedes, in decoding order, the NAL unit having transmission order index m.
A presentation timestamp (PTS) indicates the time relative to a reference clock when a picture is supposed to be displayed. A presentation timestamp is also called a display timestamp, output timestamp, and composition timestamp.
Carriage of PTS depends on the communication system and video coding standard in use. In MPEG-2 Systems, PTS can optionally be transmitted as one item in the header of a PES packet. In the JVT coding standard, PTS can optionally be carried as a part of Supplemental Enhancement Information (SEI). In ISO Base Media File Format, PTS is dedicated its own box type, Composition Time to Sample Box where the presentation timestamp is coded relative to the corresponding decoding timestamp. In RTP, the RTP timestamp in the RTP packet header corresponds to PTS.
Many of the conventional video coding standards feature the Temporal Reference (TR) coding element that is similar to PTS in many aspects. In some of the conventional coding standards, such as MPEG-2 video, TR is reset to zero at the beginning of a Group of Pictures (GOP). In the JVT coding standard, there is no concept of time in the video coding layer. The Picture Order Count (POC) is specified for each frame and field and it is used similarly to TR in direct temporal prediction of B slices, for example. POC is reset to 0 at an IDR picture.
Buffering
Streaming clients typically have a receiver buffer that is capable of storing a relatively large amount of data. Initially, when a streaming session is established, a client does not start playing the stream back immediately, but rather it typically buffers the incoming data for a few seconds. This buffering helps to maintain continuous playback, because, in case of occasional increased transmission delays or network throughput drops, the client can decode and play buffered data. Otherwise, without initial buffering, the client has to freeze the display, stop decoding, and wait for incoming data. The buffering is also necessary for either automatic or selective retransmission in any protocol level. If any part of a picture is lost, a retransmission mechanism may be used to resend the lost data. If the retransmitted data is received before its scheduled decoding or playback time, the loss is perfectly recovered.
Coded pictures can be ranked according to their importance in the subjective quality of the decoded sequence. For example, non-reference pictures, such as conventional B pictures, are subjectively least important, because their absence does not affect decoding of any other pictures. Subjective ranking can also be made on data partition or slice group basis. Coded slices and data partitions that are subjectively the most important can be sent earlier than their decoding order indicates, whereas coded slices and data partitions that are subjectively the least important can be sent later than their natural coding order indicates. Consequently, any retransmitted parts of the most important slice and data partitions are more likely to be received before their scheduled decoding or playback time compared to the least important slices and data partitions.
Pre-Decoder Buffering
Pre-decoder buffering refers to buffering of coded data before it is decoded. Initial buffering refers to pre-decoder buffering at the beginning of a streaming session. Initial buffering is conventionally done for two reasons explained below.
In conversational packet-switched multimedia systems, e.g., in IP-based video conferencing systems, different types of media are normally carried in separate packets. Moreover, packets are typically carried on top of a best-effort network that cannot guarantee a constant transmission delay, but rather the delay may vary from packet to packet. Consequently, packets having the same presentation (playback) time-stamp may not be received at the same time, and the reception interval of two packets may not be the same as their presentation interval (in terms of time). Thus, in order to maintain playback synchronization between different media types and to maintain the correct playback rate, a multimedia terminal typically buffers received data for a short period (e.g. less than half a second) in order to smooth out delay variation. Herein, this type of a buffer component is referred as a delay jitter buffer. Buffering can take place before and/or after media data decoding.
Delay jitter buffering is also applied in streaming systems. Due to the fact that streaming is a non-conversational application, the delay jitter buffer required may be considerably larger than in conversational applications. When a streaming player has established a connection to a server and requested a multimedia stream to be downloaded, the server begins to transmit the desired stream. The player does not start playing the stream back immediately, but rather it typically buffers the incoming data for a certain period, typically a few seconds. Herein, this buffering is referred to as initial buffering. Initial buffering provides the ability to smooth out transmission delay variations in a manner similar to that provided by delay jitter buffering in conversational applications. In addition, it may enable the use of link, transport, and/or application layer retransmissions of lost protocol data units (PDUs). The player can decode and play buffered data while retransmitted PDUs may be received in time to be decoded and played back at the scheduled moment.
Initial buffering in streaming clients provides yet another advantage that cannot be achieved in conversational systems: it allows the data rate of the media transmitted from the server to vary. In other words, media packets can be temporarily transmitted faster or slower than their playback rate as long as the receiver buffer does not overflow or underflow. The fluctuation in the data rate may originate from two sources.
First, the compression efficiency achievable in some media types, such as video, still images, audio and text, depends on the contents of the source data. Consequently, if a stable quality is desired, the bit-rate of the resulting compressed bit-stream varies. Typically, a stable audio-visual quality is subjectively more pleasing than a varying quality. Thus, initial buffering enables a more pleasing audio-visual quality to be achieved compared with a system without initial buffering, such as a video conferencing system.
Second, it is commonly known that packet losses in fixed IP networks occur in bursts. In order to avoid bursty errors and high peak bit- and packet-rates, well-designed streaming servers schedule the transmission of packets carefully. Packets may not be sent precisely at the rate they are played back at the receiving end, but rather the servers may try to achieve a steady interval between transmitted packets. A server may also adjust the rate of packet transmission in accordance with prevailing network conditions, reducing the packet transmission rate when the network becomes congested and increasing it if network conditions allow, for example.
Yet another advantage of pre-decoder buffering is that it enables arranging of transmission units from the reception order to decoding order. The reception order is identical to the transmission order provided that no re-order of transmission units happened in the transmission path.
Transmission of Multimedia Streams
A multimedia streaming system consists of a streaming server and a number of players, which access the server via a network. The network is typically packet-oriented and provides little or no means to guaranteed quality of service. The players fetch either pre-stored or live multimedia content from the server and play it back in real-time while the content is being downloaded. The type of communication can be either point-to-point or multicast. In point-to-point streaming, the server provides a separate connection for each player. In multicast streaming, the server transmits a single data stream to a number of players, and network elements duplicate the stream only if it is necessary.
When a player has established a connection to a server and requested for a multimedia stream, the server begins to transmit the desired stream. The player does not start playing the stream back immediately, but rather it typically buffers the incoming data for a few seconds. Herein, this buffering is referred to as initial buffering. Initial buffering helps to maintain pauseless playback, because, in case of occasional increased transmission delays or network throughput drops, the player can decode and play buffered data.
In order to avoid unlimited transmission delay, it is uncommon to favor reliable transport protocols in streaming systems. Instead, the systems prefer unreliable transport protocols, such as UDP, which, on one hand, inherit a more stable transmission delay, but, on the other hand, also suffer from data corruption or loss.
RTP and RTCP protocols can be used on top of UDP to control real-time communications. RTP provides means to detect losses of transmission packets, to reassemble the correct transmission order of packets in the receiving end, and to associate a sampling time-stamp with each packet. Among other things RTCP conveys information about how large a portion of packets were correctly received, and, therefore, it can be used for flow control purposes.
In conventional video coding standards, the decoding order is coupled with the output order. In other words, the decoding order of I and P pictures is the same as their output order, and the decoding order of a B picture immediately follows the decoding order of the latter reference picture of the B picture in output order. Consequently, it is possible to recover the decoding order based on known output order. The output order is typically conveyed in the elementary video bitstream in the Temporal Reference (TR) field and also in the system multiplex layer, such as in the RTP header.
Some RTP payload specifications allow transmission of coded data out of decoding order. The amount of disorder is typically characterized by one value that is defined similarly in many relevant specifications. For example, in the draft RTP Payload Format for Transport of MPEG-4 Elementary Streams, the maxDisplacement parameter is specified as follows:
The maximum displacement in time of an access unit (AU, corresponding to a coded picture) is the maximum difference between the time stamp of an AU in the pattern and the time stamp of the earliest AU that is not yet present. In other words, when considering a sequence of interleaved AUs, then:Maximum displacement=max{TS(i)−TS(j)}, for any i and any j>i,                 where i and j indicate the index of the AU in the interleaving pattern and TS denotes the time stamp of the AU        
It has been noticed in the present invention that in this method there are some problems:
RTP timestamp indicates the capture/display timestamp. The JVT coding standard allows decoding order different from output order. The receiver buffer is used to reorder packets from transmission/reception order to decoding order. Thus, displacement specified between differences in RTP timestamps cannot be used to arrange transmission units from transmission order to decoding order.
The U.S. patent application 60/483,159 describes buffering operation based on parameter sprop-interleaving-depth, which specifies the maximum number of VCL NAL units that precede any VCL NAL unit in the NAL unit stream in transmission order and follow the VCL NAL unit in decoding order. Constant N is the value of the sprop-interleaving-depth parameter incremented by 1. If the parameter is not present, a 0 value number could be implied. The receiver buffering operates as follows.
When the video stream transfer session is initialized, the receiver allocates memory for the receiving buffer for storing at least N pieces of VCL NAL units. The receiver then starts to receive the video stream and stores the received VCL NAL units into the receiving buffer, until at least N pieces of VCL NAL units are stored into the receiving buffer.
When the receiver buffer contains at least N VCL NAL units, NAL units are removed from the receiver buffer one by one and passed to the decoder. The NAL units are not necessarily removed from the receiver buffer in the same order in which they were stored, but according to the decoding order number (DON) of the NAL units, as described below. The delivery of the packets to the decoder is continued until the buffer contains less than N VCL NAL units, i.e. N−1 VCL NAL units.
It has been noticed that the buffering operation in the U.S. patent application 60/483,159 has some problems. For example, if there are transmission losses the decoder may not receive all the transmitted transmission units. Therefore, the decoding buffer is filled more slowly in the decoder than in a situation in which all transmission units are received. Thus, the pictures may be output from the decoder buffer slower than what is optimal for the decoder. Another problem may arise when consecutive decoding order numbers in transmission order do not follow a constant pattern. In other words, if the difference of decoding order numbers of two successively decodable transmission units changes from time to time then sprop-interleaving-depth is selected according the largest number of VCL NAL units in the receiver buffer that must be present in order to arrange NAL units correctly in decoding order and therefore buffering for an individual NAL unit may last longer than necessary to output it from the receiver buffer in correct decoding order.